Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences

[Jan Kara <jack@suse.cz>]

On Sat 06-02-16 23:43:49, Ross Zwisler wrote:
> On Sun, Feb 07, 2016 at 10:40:53AM +1100, Dave Chinner wrote:
> > On Fri, Feb 05, 2016 at 03:25:00PM -0700, Ross Zwisler wrote:
> > > On Thu, Feb 04, 2016 at 09:29:57PM +0100, Jan Kara wrote:
> > > > I think changes aren't very intrusive so we can feed them in during RC
> > > > phase and frankly, you have to move to using ->writepages() anyway to make
> > > > sync(2) work reliably.
> > > 
> > > I've been looking into this a bit more, and I don't think we actually want to
> > > have DAX flushing in response to sync(2) or syncfs(2).  Here's some text from
> > > the BUGS section of the sync(2) man page:
> > > 
> > > BUGS
> > > 	According to the standard specification (e.g., POSIX.1-2001), sync()
> > > 	schedules the writes,  but  may  return before  the  actual  writing
> > > 	is done.  However, since version 1.3.20 Linux does actually wait.
> > > 	(This still does not guarantee data integrity: modern disks have large
> > > 	caches.) 
> > > 
> > > Based on this I don't believe that it is a requirement that sync and syncfs
> > > actually flush the data durably to media before they return - they just need
> > > to make sure it has been sent to the device, which is always true for all
> > > writes PMEM via DAX, even without any calls to dax_writeback_mapping_range().
> > 
> > That's an assumption we've already pointed out as being platform
> > dependent, not to mention also being undesirable from a performance
> > point of view (writes are 10x slower into pmem than into the page
> > cache using the same physical memory!).
> > 
> > Further, the ordering constraints of modern filesystems mean that
> > they guarantee durability of data written back when sync() is run.
> > i.e.  ext4, XFS, btrfs, etc all ensure that sync guarantees data
> > integrity is maintained across all the data and metadata written
> > back during sync().
> > 
> > e.g. for XFS we do file size update transactions at IO completion.
> > sync() triggers writeback of data, then runs a transaction that
> > modifies the file size so that the data is valid on disk. We
> > absolutely need to ensure that this transaction is durable before
> > sync() returns, otherwise we lose that data if a failure occurs
> > immediately after sync() returns because the size update is not on
> > disk.
> > 
> > Users are right to complain when data written before a sync() call
> > is made does not accessible after a crash/reboot because we failed
> > to make it durable. That's why ->sync_fs(wait) is called at the end
> > of the sync() implementation - it enables filesystems to ensure all
> > data and metadata written during the sync processing is on durable
> > storage.
> > 
> > IOWs, we can't language-lawyer or weasel-word our way out of
> > providing durability guarantees for sync().
> 
> To be clear, we are only talking about user data that was mmaped.  All I/Os
> initiated by the filesystem for metadata, and all user issued I/Os will be
> durable on media before the I/O is completed.  Nothing we do or don't do for
> fsync, msync, sync or syncfs() will break filesystem metadata guarantees.

Yes, but be careful here:

So far ext4 (and AFAIK XFS as well) have depended on the fact that
blkdev_issue_flush() (or WRITE_FLUSH request) will flush all the volatile
caches for the storage. We do such calls / writes from transaction commit
code to make sure that all metadata *and data* writes reach permanent
storage before we make some metadata changes visible. This is necessary so
that we don't expose uninitialized block contents after a power failure
(think of making i_size increase / block allocation visible after power
failure without data being durable). 

For PMEM, we ignore blkdev_issue_flush() / WRITE_FLUSH requests on the
grounds that writes are either done bypassing caches (the case for metadata
IO and IO done via dax_do_io()). Currently we are fine even for mmap
because both ext4 & XFS zero out allocated blocks using non-cached writes
so even though latest data needn't be on persistent storage we won't expose
stale data. But it is a catch that may hit us in future. So from this POV
flushing caches from ->writepages() would also make PMEM give more similar
guarantees to fs as ordinary block storage.

> I think the question then is: "Do users currently rely on data written to an
> mmap to be durable on media after a sync() or syncfs(), in spite of the BUGS
> section quoted above?"
> 
> If the answer for this is "yes", then I agree that we need to enhance the
> current DAX fsync code to cover the sync use case.

IMO the answer is yes.

> However, as stated previously I don't think that it is as easy as just moving
> the call to dax_writeback_mapping_range() to the sync() call path.  On my
> system sync() is called every 5 seconds, and flushing every page of every DAX
> mmap that has ever been dirtied every 5 seconds is unreasonable.

It is not sync() that is called but background writeback. But you are
correct that ->writepages() will get called for a dirty inode every 5
seconds.

> If we need to support the sync() use case with our DAX mmap flushing code, I
> think the only way to do that is to clear out radix entries as we flush them,
> so that the sync calls that happen every 5 seconds are only flushing new
> writes.

I agree and I forgot about the issue that we currently don't clear out
dirty radix tree entries for DAX. I also agree that clearing of written
radix tree entries needs to be implemented before flushing caches from
->writepages() is practical.

> I think we're actually pretty far from this, at least for v4.5.  The issue is
> that I don't think we can safely clear radix tree dirty entries during the DAX
> code that is called via ->writepages().  To do this correctly we would need to
> also mark the PTE as clean so that when the userspace process next writes to
> their mmap mapping we would get a new fault to make the page writable. This
> would allow us to re-dirty the DAX entry in the radix tree.

Yup, so I agree that clearing of radix tree entries probably is not a 4.5
material.

> I implemented code to do this in v2 of my set, but ripped it out in v3:
> 
> https://lkml.org/lkml/2015/11/13/759 (DAX fsync v2)
> https://lkml.org/lkml/2015/12/8/583  (DAX fsync v3)
> 
> The race that compelled this removal is described here:
> 
> https://lists.01.org/pipermail/linux-nvdimm/2016-January/004057.html
> 
> If this is really where we need to be, that's fine, we'll have to figure out
> some way to defeat the race and get this code into v4.6.

I think we need to find a way to safely clear radix tree entries anyway.
And I agree that without page lock it is not easy. I see two possibilities
here:

1) Protect dax_writeback_mapping_range() with exclusive
EXT4_I(inode)->i_mmap_sem and provide new callback called from
wp_page_reuse() to mark index dirty in the radix tree (that would take
shared i_mmap_sem). This is relatively expensive but it should work.

Or

2) We could devote one bit in the radix tree exceptional entry as a bitlock
equivalent to page lock. This will likely scale better but it is more
intrusive.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR